1. FIELD OF THE INVENTION
The present invention relates to phosphorous steel powder mixtures to be used within the powder metallurgy. In addition to iron and phosphorus these powder mixtures can contain other alloying elements common within this technique, such as copper, nickel, molybdenum, chromium and carbon.
2. DESCRIPTION OF THE PRIOR ART
The use of phosphorus as an alloying element within the powder metallurgy has been known since the forties. Sintered steel alloyed with phosphorus has substantially improved strength characteristics in relation to non-alloyed sintered steel. Already at an early date there were for this object used mixtures of pure iron powder and ferrophosphorus powder. However, the ferrophosphorus first used had a composition which made it extremely hard and caused a considerable wearing of the tools. This drawback has been reduced to an acceptable degree by using a ferrophosphorus powder having a lower content of phosphorus and thereby reduced hardness see for example Swedish Pat. No. 372,293.
However, sintered details manufactured by pressing and sintering such steel powder mixtures sometimes have an unacceptable brittleness. This is revealed for example by the fact that a population of sintered test bars made from these mixtures can comprise individuals having extremely reduced mechanical characteristics especially with regard to impact strength and permanent strain after rupture (break elongation). As the advantage of phosphorus alloyed sintered steels is high strength in combination with very good strain characteristics the above brittleness risks are very serious.
Said brittleness risk has shown up to be present when the ferrophosphorus is of such composition that there is established a liquid phase at the sintering temperature. At the usually used sintering temperatures, 1040.degree. C and above that, this fact provides that phosphorus contents of more than 2.8% in the ferrophosphorus give a sintered material having an increased brittleness risk. The fact that ferrophosphorus having a high phosphorus content is used in spite of this drawback is dependent on the favorable sintering process which is provided by the liquid phase and the favorable distribution of the phosphorus in turn providing for a rapid indiffusion thereof which is obtained because of the fact that the ferrophosphorus provides for a liquid phase.